SHALLOW SUBSURFACE CHARACTERIZATION OF SEISMICALLY ACTIVE NARMADA-SON FAULT USING GPR, GUJARAT, WESTERN INDIA

The Narmada-Son Fault (NSF) is a major E-W trending crustal scale seismically active fault in the central part of the Indian plate. The results of Ground Penetrating Radar (GPR) surveys and field based studies along the Narmada Son Fault (NSF) in the western part and its neotectonic implications are presented here. Geomorphologically, the fault is prominently expressed as north facing and ENE-WSW trending scarps developed in the Deccan Traps, Tertiary sedimentary rocks and Pleistocene sediments. The scarps separate the alluvial terrain to the north and the rugged mountainous region to the south. The NSF is traversed by several transverse faults displacing the scarpline. On the basis of transverse faults, the NSF in the study area is divisible into four morphotectonic segments bearing different geomorphic set up which is further related to variable magnitude of tectonic activity laterally along the length of NSF.

The GPR survey carried out across the NSF at several sites in all four segments to laterally map the near surface trace and the shallow subsurface nature of the fault. GPR surveys, in segment I, II and III were carried out over the Deccan traps comprise of various basaltic flows. While in segment IV, one profile conducted over the Tertiary sedimentary rocks and the other is over the Quaternary sediments. Although the host material is same over which the survey conducted, the faulting activities have indirectly changed the properties of the rocks by shearing and crushing caused during the fault movements. This has made easy to identify fault zone and to locate the master fault within that zone. In the GPR profiles, sheared rocks show low amplitude reflection pattern. And the displacement within such zone made possible to locate the master fault, i.e. NSF. The NSF is found to be a steep vertical normal fault that becomes reveres near the surface, whereas at places the displacement is along almost vertical plane. The GPR data along with field evidences of neotectonic activity that include stratigraphic development of Quaternary sediments and variable incision suggest rejuvenation of the area under dominant N-S oriented compressional stress environment.